Thermostatically controlled heating implement

ABSTRACT

A thermostatically controlled heating implement in which one surface of a formed sheet of metal defines the heating surface of the implement and in which the formed sheet forms one element in a bimetal bow-type thermomechanical switch to control the heating temperature of the implement.

United States Patent 1191 [111 3,719,798

Nelson et al. [451 March 6, 1973 541 THERMOSTATICALLY CONTROLLED 2,640,130 5/1953 Reingruber et a1. ..337/398 x HEATING IMPLEMENT I 1,907,199 5/1933 [75] Inventors: Donald D. Nelson, Bloomington; Thomas S. Shevlin, White Bear Lake, both of Minn. FOREIGN PATENTS OR APPLICATIONS 1 1 Assignee: n e ota Mining and Mflnufactur- 310,581 5/1929 Great Britain ..337/396 ing Company, Saint Paul, Minn. 22 Filed: Feb. 19 1971 Primary Examiner-George Harris Assistant Examiner-F. E. Bell [21] Attorney-Kinney, Alexander, Sell, Steldt & Delahunt 52 us. c1 ..219/s12,337/39s 7] ABSTRACT 51 int. Cl. ..H05b 1/02 58 Field of Search ..219/512, 441,442, 512; A thermstat'cany controlled heatmg Implement m which one surface of a formed sheet of metal defines 337/399 398 the heating surface of the implement and in which the I formed sheet forms one element in a bimetal bow-type [56] References Cned thermomechanical switch to controlthe heating tem- UNITED STATES PATENTS perature of the implement. 2,537,808 1/1951 Bate ..219/512 X 3 Claims, 4 Drawing Figures PATENTEDHAR 61m 3.719.798

"a w mm 16 W ,4 F/VEYS' THERMOSTATICALLY CONTROLLED HEATING IMPLEMENT FIELD OF THE INVENTION The present invention relates to a thermostatically controlled heating implement.

BACKGROUND OF THE INVENTION The prior art is replete with bimetal bow-type thermomechanical thermostats as exemplified by U.S. Pat. Nos. 2,389,686; 2,584,924; and 2,703,832. In the thermostats disclosed in those patents the bow member is formed of a metal having a low coefficient of thermal expansion and the bow member is secured at its ends to a flat bar of a metal having a high coefficient of thermal expansion, such as brass or aluminum. Generally, one surface of a formed sheet of metal defines the heating surface of the implement and the flat bar of the thermostat is mounted on the opposite surface of the formed sheet in such a manner as to permit expansion and contraction of the flat bar and the bow member independent of any expansion or contraction of parts of the implement. Such thermostats have had limited application since the implement must exchange heat with the flat bar of the thermostat before the thermostat will respond to changes in temperature of the heating surface of the implement. Thus, the responsiveness of the thermostat to the temperature of the heating surface of the implement is dependent upon heat transfer between the heating surface of the implement and the flat bar of the thermostat. Due to the thermal mass of the formed sheet of the implement, the thermal mass of the flat bar and the imperfect thermal coupling between the formed sheet and the flat bar, the flat bar will necessarily heat the cool more slowly than the heating surface of the implement. For this reason, as the implement is brought up to temperature, the temperature of the heating surface of the implement may greatly overshoot that temperature preset on the thermostat. Furthermore, due to this necessary heat transfer such thermostats are relatively slow to respond to temperature changes of the heating surface of the implement.

SUMMARY OF THE INVENTION The thermostatically controlled heating implement of the present invention overcomes the aforementioned disadvantages of the prior art thermostats combined with a heating implement and comprises a frame including a formed sheet of a metal having a relatively high coefficient of thermal expansion, one surface of the formed sheet defining a heating surface. The formed sheet is heated to heat substances contacting the heating surface thereof. A strip of metal having a relatively low coefficient of thermal expansion is rigidly secured at its ends to the surface of the formed sheet opposite the heating surface, the points of securement of the metal strip being spaced a lesser distance than the length of the metal strip between them to bow the metal strip away from the surface of the formed sheet. Heating of the formed sheet is controlled in response to changes in the bowing of the metal strip to cause heating of the formed sheet when the bowing of the metal strip is greater than a predetermined minimum extent which corresponds to a preset temperature. When the temperature of the formed sheet is below the preset temperature it has lesser dimensions, the strip of metal is bowed away from the surface of the formed sheet greater than the predetermined minimum extent and the heater control activates the heater. As the formed sheet is heated it expands thereby causing the metal strip to be less bowed and when the temperature of the formed sheet reaches the preset temperature the metal strip is bowed exactly the predetermined minimum extent and the heater is deactivated. Thus, thermal input to the formed sheet is controlled directly in accordance with the expansion and contraction of the formed sheet and thereby directly according to the temperature of the heating surface of the implement. The thermostatically controlled heating implement of the present invention therefore, permits its heating surface to be rapidly heated to a preset temperature without overshoot and it is very responsive to changes in the temperature of its heating surface.

THE DRAWING In the drawing:

FIG. I is an elevation view of a thermostatically controlled heating implement made in accordance with the present invention and partially in section;

FIG. 2 is a bottom view of the implement of FIG. 1 with parts removed for clarity;

FIG. 3 is a cross-sectional view taken generally along line 3--3 of FIG. 2; and

FIG. 4 is a graphical representation of the temperature response of the thermostatically controlled heating implement of FIG. 1.

Referring now to the drawing there is illustrated a thermostatically controlled heating implement, generally designated 10, constructed in accordance with the present invention.

In the illustrated embodiment of the present invention the implement in a cooking vessel comprising a bowl 12 formed from a sheet of metal having a relatively high coefficient of thermal expansion, for example, a low carbon steel giving said bowl a uniform thin wall. The inner surface 14 of the bowl 12 defines a heating surface for preparing food stuffs placed in the bowl. A pair of threaded studs 16 are secured to the exterior surface of the bowl 12, for example by welding, at precisely .located positions equally spaced from the lowest point of the bowl.

An elongated rectangular strip 18 of a metal having a relatively low coefficient of thermal expansion, for example Invar or Kovar, is formed with an aperture near each of its ends which apertures are larger than the diameter of the threaded studs 16. The metal strip 18 is bowed centrally of its length to space its central portion away from the exterior surface of the bowl I2 and to fit each of its end apertures over one of the threaded studs A U-shaped contact support frame 20 is formed to fit over the bowed strip 18 and has apertured end flanges formed to fit over the studs 16 for securing to the bowl 12. A nut 22 of a deformable metal, for example copper, is threaded onto each stud 16 into the apertures in the bowed strip 18 and the contact support frame 20 until it deforms into those apertures to rigidly secure the ends of the bowed strip 18 to the bowl 12. The portion of the contact support frame 20 over the bow 18 is flat and it carries a pair of electrical connecting posts 24 and 25 which are appropriately electrically insulated from the support frame 20. A first contact support arm 2" I, formed of spring steel, is secured to one of the posts 24, extends therefrom over the central portion of the bow l8 and carries an electrical contact 28 at its free end. A second contact support arm 29, also formed of spring steel, is secured to the connecting post 25, extends therefrom in the same direction as the first arm 27 extends from the post 24 and carries a second electrical contact 30 at its free end in aligned opposed relationship to the contact 28. Centrally of its length, the bow 18 adjustably supports a ceramic tip 19. The first contact support arm 27 is biased against the ceramic tip 19 on the bow 18 while the second contact support arm 29 is biased against a ceramic tip 33 carried by a temperature adjustment stud 32.

The temperature adjustment stud 32 is threaded through the contact support frame 20 near the free end of the second contact support arm 29 and it carries its ceramic tip 33 into contact with the second support arm 29 for adjusting the gap between contacts 28 and 30. The adjustment stud 32 is formed with a flange 34 which cooperates with a stop 36 extending from the contact support frame 30 to limit rotation of the adjustment stud 32. An adjustment knob is secured to the exposed end of the adjustment stud.

A resistance heater 38, for example having a silverfrit composition, is secured to the exterior surface of the bowl 12 in a serpentine pattern. The serpentine pattern of the heater 38 is continuous on the portion of the bowl 12 hidden in FIG. 1 so that the heater is symmetrical with respect to the bow 18 and the contact support frame 20. A lead wire 41 is connected between one end of the heater 38 and the electrical connecting post 25. A two wire power cord 39 has one of its lead wires connected to the opposite end of the heater 38 and its other wire connected to the connecting post 24 to provide a series connection between the heater 38 and the electrical contacts 28 and 30.

The temperature adjustment is calibrated by rotating the adjustment knob 35 to close the contacts 28 and 30 while sensing the temperature of the inner surface 14 of the bowl 12, for example with a thermistor. Selected temperatures sensed by the thermistor may then be marked on the visible portion of the adjustment knob 35 in alignment with the stop 36.

In use, the temperature adjustment knob 35 is rotated to the desired temperature setting for heating foodstuffs contained in the bowl 12 thereby closing the contacts 28 and 30. Closing of the contacts completes the electrical heating circuit from the power cord through the heater, through the thermostat and back to the power cord thereby supplying power to the heater 38 to raise the temperature of the bowl 12. As the bowl 12 is heated it expands thereby increasing the distance between the studs 16 and the ends of the bowed strip 18 and decreasing the extent of the bowing of the strip. When the temperature of the bowl 12 reaches that temperature preset on the adjustment knob 35 the bowed strip 18 will be bowed a predetermined minimum extent corresponding to the preset temperature and will have retracted its central portion sufficiently toward the bowl 12 to permit the contacts 28 and 30to open the electrical circuit and shut off power to the heater 38.

In FIG. 4 there is illustrated a time-temperature response curve for one specific embodiment of the device constructed according to the present invention in which the formed sheet of the implement is constructed of a low carbon steel and the bowed strip 18 is a strip of Kovar. As can be seen from the graph the temperature of the heating surface rapidly approaches the preset temperature of Centigrade and as it approaches that preset temperature it levels off and creeps up to the preset temperature without overshooting. Further experiments with a bowl l2 constructed of a low carbon steel and a bow strip 18 constructed of Kovar have shown that this structure is extremely temperature sensitive. When the empty bowl was brought up to temperature so that the contacts 28 and 30 were open, the operator, by blowing on the inner surface of the bowl 12, caused the contacts 28 and 30 to close to again supply power to the heater 38.

Having thus described the invention, what is claimed 1. A thermostatically controlled heating implement comprising:

a frame including a formed sheet of metal having a relatively high coefficient of thermal expansion, one surface of said formed sheet defining a heating surface of said implement,

means for heating said formed sheet to heat substances contacting said heating surface thereof,

a strip of metal having a relatively low coefficient of thermal expansion rigidly secured at its ends to the surface of said formed sheet opposite said heating surface, said points of securement of said metal strip being spaced a lesser distance than the length of said metal strip between them to bow said metal strip away from said opposite surface of said formed sheet, and

means for controlling said means for heating in response to changes in the bowing of said metal strip to cause heating of said formed sheet when the bowing of said metal strip is greater than a predetermined minimum extent,

whereby when the temperature of said formed sheet is below a preset temperature it has lesser dimensions, said strip of metal is bowed away from said opposite surface of said formed sheet greater than said predetermined minimum extent and said means for controlling activates said means for heating; as said formed sheet is heated it expands thereby causing said metal strip to be less bowed and when the temperature of said formed sheet reaches said preset temperature said metal strip is bowed said predetermined minimum extent and said means for heating is deactivated.

2. The implement of claim 1 wherein said means for heating and said means for controlling said means for heating comprise an electrical heating circuit including electrical contact means supported from said opposite surface of said formed sheet and extending over said bowed strip of metal at a fixed operating distance from said opposite surface for closure by said strip of metal to complete said electrical heating circuit when said strip of metal is bowed away from said opposite surface of said formed sheet greater than said predetermined minimum extent.

3. The implement of claim 2 including manually adjustable means for adjusting said fixed operating distance of said electrical contact means to permit adjustment of said predetermined minimum bowing of said metal strip at which said metal strip closes said contact means thereby to permit adjustment of said preset temperature. 5 

1. A thermostatically controlled heating implement comprising: a frame including a formed sheet of metal having a relatively high coefficient of thermal expansion, one surface of said formed sheet defining a heating surface of said implement, means for heating said Formed sheet to heat substances contacting said heating surface thereof, a strip of metal having a relatively low coefficient of thermal expansion rigidly secured at its ends to the surface of said formed sheet opposite said heating surface, said points of securement of said metal strip being spaced a lesser distance than the length of said metal strip between them to bow said metal strip away from said opposite surface of said formed sheet, and means for controlling said means for heating in response to changes in the bowing of said metal strip to cause heating of said formed sheet when the bowing of said metal strip is greater than a predetermined minimum extent, whereby when the temperature of said formed sheet is below a preset temperature it has lesser dimensions, said strip of metal is bowed away from said opposite surface of said formed sheet greater than said predetermined minimum extent and said means for controlling activates said means for heating; as said formed sheet is heated it expands thereby causing said metal strip to be less bowed and when the temperature of said formed sheet reaches said preset temperature said metal strip is bowed said predetermined minimum extent and said means for heating is deactivated.
 1. A thermostatically controlled heating implement comprising: a frame including a formed sheet of metal having a relatively high coefficient of thermal expansion, one surface of said formed sheet defining a heating surface of said implement, means for heating said Formed sheet to heat substances contacting said heating surface thereof, a strip of metal having a relatively low coefficient of thermal expansion rigidly secured at its ends to the surface of said formed sheet opposite said heating surface, said points of securement of said metal strip being spaced a lesser distance than the length of said metal strip between them to bow said metal strip away from said opposite surface of said formed sheet, and means for controlling said means for heating in response to changes in the bowing of said metal strip to cause heating of said formed sheet when the bowing of said metal strip is greater than a predetermined minimum extent, whereby when the temperature of said formed sheet is below a preset temperature it has lesser dimensions, said strip of metal is bowed away from said opposite surface of said formed sheet greater than said predetermined minimum extent and said means for controlling activates said means for heating; as said formed sheet is heated it expands thereby causing said metal strip to be less bowed and when the temperature of said formed sheet reaches said preset temperature said metal strip is bowed said predetermined minimum extent and said means for heating is deactivated.
 2. The implement of claim 1 wherein said means for heating and said means for controlling said means for heating comprise an electrical heating circuit including electrical contact means supported from said opposite surface of said formed sheet and extending over said bowed strip of metal at a fixed operating distance from said opposite surface for closure by said strip of metal to complete said electrical heating circuit when said strip of metal is bowed away from said opposite surface of said formed sheet greater than said predetermined minimum extent. 